Research of the Mode Instability Threshold in High Power Double Cladding Yb‐doped Fiber Amplifiers

Abstract

We experimentally investigate the behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier is pumped by broad linewidth laser diodes and narrow linewidth laser diodes respectively. It is found that the MI threshold increases by 26% when the amplifier is pumped by the broad linewidth laser diodes. Experiment results show that the MI threshold is affected by the local heat load rather than the average or the total heat load. The calculation shows that the local heat deposit actually plays the key role to stimulate the MI behaviour. At the MI threshold position in the fiber, the local heat deposit also changes dramatically. The effect of the thermal conductivity on the MI threshold is also studied. Our investigation shows that the MI threshold increases from 1269 W to 1950 W when the thermal conductivity of the fiber amplifier is increased from 0.3 W/(m · K) to 5 W/(m · K). Through optimizing the pump linewidth and the cooling efficiency of the gain fiber, the MI threshold is doubled in our experiment.

Impact of mode instability on polarization extinction ratio in backward pumped fiber amplifiers

“…The influence of the MI effect on PER of backward-pumped PM fiber amplifier has been investigated in the experiment, and the results show that the MI effect has little impact. The phenomena is different from the results in [14,15], where the MI effect can result in the decreasing of PER in the forward-pumped PM fiber amplifier.…”

“…The MI effect is originated from the thermal effect, which can result in the dynamic power coupling among the fundamental mode (FM) and high order modes (HOMs), and thereby dramatically deteriorate the beam quality of fiber laser and limit the output power of PM fiber laser. Even worse, recent experimental results in forward-pumped PM fiber lasers express that the MI effect can also lead to the decreasing of PER of PM fiber laser, which further limiting the application of PM fiber lasers [14,15]. However, the physical mechanism of MI induced degradation of PER in forward-pumped PM fiber laser has not been analyzed.…”

2.5 kW Narrow Linewidth Linearly Polarized All-Fiber MOPA With Cascaded Phase-Modulation to Suppress SBS Induced Self-Pulsing

“…For the NLE, using large mode area (LMA) and short length fiber [13], [14], counter pumping structure [15]- [17], phase modulated seed [18]- [21] are effective suppressing techniques. As for the MI, the various optimizing methods have been presented [22]- [25]. By now, the output power of all-fiber narrow-linewidth non-PM fiber laser has achieved 3.7 kW with a linewidth of 79 GHz [21].…”

Towards the Enhancement of the TMI Threshold in Monolithic High-Power Fiber System by Controlling the Pump Distribution and the Seed Power

“…As discussed above, for both the pump distribution-dependent and the seed power-dependent TMI thresholds, the simulation results show good agreements with the experimental data. These results powerfully predict the promising potential of this monolithic high-power fiber laser system for more than 3 kW power scaling given that some other TMI mitigation strategies are combined, for example, enlarging the core/inner cladding diameter ratio of the active fiber, reducing the numerical aperture or the dopant concentration of the active fiber, reforming the cooling system to lower the local heat load of the active fiber [49], increasing the linewidth of the seed laser [48], [64], stabilizing the laser system to lower the noise [65], optimizing the bending of the active fiber [66], and so on.…”

“…The active fiber-based ones included new designs of the active fiber [23], [25], [41], [42], optimizing numerical aperture [24], [27] or dopant concentration of the active fiber [43], [44], enlarging core/inner cladding diameter ratio of the active fiber [17], [45], optimizing bend diameter of the active fiber [24], [27], [46] and so forth. Then the system-based ones referred to shifting pump or signal wavelength [39], [47], changing the linewidth of the signal laser or pump laser [48], [49], controlling the excitation of mode content at injection end of the amplifier [22], adapting the cooling condition [20], [49], and so on. The recent research even disclosed that TMI was related to the occurrence of the nonlinear effects such as the stimulated Brillouin scattering (SBS) [50] and the stimulated Raman scattering (SRS) [51], [52], which provided new cognitions to suppress the TMI in terms of optimizing the system configuration to prevent the nonlinear effects.…”